The present invention relates to a glow plug as a preheat plug for improving the starting characteristics of a diesel engine and, more particularly, to an improvement in a self temperature control type glow plug having excellent fast heating and self temperature saturation properties and capable of achieving long-time afterglow.
Glow plugs having various structures have been known as glow plugs for improving the starting characteristics of a diesel engine. The applicant of the present invention has proposed a self temperature control type glow plug in Japanese Patent Laid-Open No. 57-182026. In this glow plug, a fast heating function and a temperature saturation function for preventing overheating of heating wires and obtaining stable heating characteristics are realized by an elaborate combination of resistor elements consisting of two types of materials.
More specifically, a glow plug of this type has a structure in which a first resistor element as a heating element and a second resistor element connected in series with the first resistor element and consisting of a material having a larger positive resistance-temperature coefficient than the first resistor element are embedded in a heat-resistant insulating powder in a metal sheath. In addition, a gap for delaying heat transfer from the first resistor element is set between the two resistor elements so as to ensure a fast heating property by supplying large power to the first resistor element immediately after energization to quickly cause it to generate heat. In addition, since the resistance of the second resistor element is increased with an increase in temperature thereof after the lapse of a predetermined period of time, the power supply to the first resistor element is reduced, thus performing the self temperature saturation function for preventing disconnection and the like upon fusing caused by overheating of the first resistor element. With such a structure, since a temperature control means or the like for controlling power to be supplied is not required to be arranged on an energization circuit for the glow plug, the cost of the overall preheat unit can be decreased.
In such a conventional glow plug, although a fast heating function and a self temperature saturation function can be ensured to a certain degree, it is difficult to obtain a heating property in which a heat temperature is decreased during afterglow after the start of an engine. That is, although the conventional glow plug can perform afterglow of about several tens seconds, a recent requirement of long-time (ten minutes or more) afterglow cannot be satisfied. In order to achieve the above-mentioned fast heating function and to perform a long-time afterglow operation while decreasing the heat temperature, a relay used during a heating period and a relay used during an afterglow period must be separately incorporated in the energization circuit for the glow plug, and a voltage-drop resistor or the like must be incorporated in the circuit on the afterglow side. This causes increases in the number of circuit components and the cost of the overall unit.
In order to achieve long-time afterglow by using only the glow plug without adding any element on the circuit, a self temperature control function is required. According to this function, energization power to the heating element is self-controlled to greatly improve the heating characteristics and prevent overheating at a heater portion. At the same time, in order to ensure durability of the heating wires, the saturation temperature is decreased below a proper temperature and is maintained at this temperature. A great demand has, therefore, arisen for a glow plug having a heater portion which has excellent fast heating and self temperature saturation properties and is highly reliable in terms of heat resistance.
In addition, other conventional glow plugs constituted by a combination of two types of heating wire as described above have been proposed in, e.g., Japanese Patent Laid-Open Nos. 54-60630 and 57-87535. In these glow plugs, in order to achieve a fast heating function, the diameter of a sheath front end portion in which a front end heating wire as a heating portion is embedded is set be small to have a smaller heat capacity than that of a portion in which a rear end heating wire constituting a control portion is embedded. In these conventional structures, the function for obtaining the first heating property can be achieved to a certain degree, in which large power is supplied to the front end heating wire in an initial energization period to obtain a predetermined heat temperature. However, the overshoot function for decreasing the heat temperature after the lapse of a predetermined period of time, and performing long-time afterglow while ensuring the durability of the heating wires cannot be satisfactorily achieved. That is, with the above-described structures, the conventional glow plugs cannot obtain the required characteristics that a heat temperature is temporarily increased to a required temperature, and the heat temperature is sufficiently decreased with the lapse of time to be saturated. Therefore, a measure for satisfying the above-described requirements in consideration of these points is required.